More often than not I see the iPhone being used with two hands … not too unlike the commercials and demo videos actually. Outside of the iPod function, the iPhone does feel more comfortable to use with both hands. The G1 in my limited use, also seemed like it wanted me to use two hands – one to hold and the other to tap / slide.
When we were at Nokia World a few weeks ago, the product team made it very clear that a substantial design consideration of the upcoming N97 was to make sure it could be used one-handed. Clearly this is when closed vs. opened as you’d want both hands for the slide out QWERTY keyboard. The Nokia 5800 Express Music also seems optimized for single handed use which makes sense given it’s similar size and shape to the N97.
While I’m not sure how critical it is either way, one-handed mobile usage is certainly a great option to have when you are actually moving around. One-handed use almost seems more “power user” to me over the two-handed option…. perhaps like touch typing vs hunt and peck. I find that for the most part I do things with a single hand though as I’ve thought about this and observed my behavior a bit, my other hand does tend to pop up when I am reading for a longer period or on the train. What’s your usage?